1. Field of the Invention
The present invention relates to a structure for bonding a semiconductor device to a substrate and, more particularly, to a structure in which a semiconductor device is bonded to a substrate by bonding a bump provided on an electrode of the semiconductor device to a bump provided on an electrode of the substrate.
2. Description of Related Art
Referring to FIG. 1A, a metal connecting terminal 104 formed by an Au ball is provided on a bonding pad 107 of a semiconductor element 105. Provided on a substrate pad 111a of a substrate 111 is a metal section 112 formed by solder. When bonding the metal connecting terminal 104 and the metal section 112 face down, Au which forms the metal connecting terminal 104 is diffused in the metal section 112 to produce an Au-Sn eutectic alloy. The pattern of the substrate pad 111a is generally circular or square. A joint 113 between the metal connecting terminal 104 and the metal section 112 bulges in the middle thereof like a barrel as shown in FIG. 1B in the process of the melting and solidifying of solder. The section of the joint 113 obtained by cutting it horizontally is circular. Such a conventional structure wherein a semiconductor device is bonded to a substrate has been disclosed, for example, in Japanese Unexamined Patent Publication No. 62-281435, "THE DESIGN OF FLIPCHIP JOINT BY OTHER METAL BUMP--FLIP CHIP ATTACH TECHNOLOGY" (IMC 1994 Proceeding, Omiya, Apr. 20 to Apr. 22, 1994), and "THE DESIGN AND RELIABILITY OF FLIP CHIP ATTACH JOINT ON SURFACE LAMINAR CIRCUIT" (ISHM '93 Proceedings).
In the conventional connecting structure, since the joint between the semiconductor element and the substrate is shaped like a barrel, the stress due to the mismatch in coefficient of thermal expansion between the semiconductor element 105 and the substrate 111 is concentrated on the joint between the bonding pad 107 and the metal connecting terminal 104 or on the joint between the substrate pad 111a and the metal section 112, leading to a shortcoming that breakage is apt to happen at these joints. In particular, the joint between the substrate pad with low rigidity and the metal section is vulnerable.
Further, molten solder flows out of the substrate pad; therefore, when the interval between adjacent substrate pads which are disposed with a narrow pitch is decreased, a solder bridge is likely to be formed, frequently causing contact between adjacently disposed substrate pads through solder.